Standard ADSL is a standard defining the bidirectional transmission of digital data between a subscriber and an exchange center, which in particular provides that the exchange center can provide data to the user in a frequency range between 138 kHz and 1 MHz with a rate ranging to 8 megabits per second. This standard also provides that the user can transmit data to the exchange center in a frequency range between 30 kHz and 138 kHz, with a rate up to 1 megabit per second.
FIG. 1 shows in the form of blocks a circuit 2 enabling the computer of a subscriber (not shown) connected to a modem 4 to exchange information according to standard ADSL with an exchange center (not shown) via a twin-wire bidirectional line 6. Modem 4 has an output connected to a twin-wire transmit line 8 and an input connected to a twin-wire receive line 10. The transmit and receive lines are coupled to transmission line 6 via a two-wire/four-wire coupling means 12 commonly called a hybrid converter. A first amplifier 14 is interposed between the modem output and the transmit line and a second amplifier 16 is interposed between the receive line and the modem input. Resistors R1 and R2 in series with the outputs of amplifier 14 adapt the impedance of transmit line 8 to the impedance of transmission line 6.
According to standard ADSL, when no information is exchanged via transmission line 6, circuit 2 is in a so-called “stand-by” state, waiting for a communication. Any communication is preceded by a start-of-communication signal, or call tone, formed of several pure frequency signals. Circuit 2 must include a start-of-communication signal detector, which arouses it from its stand-by state. Conventionally, the detector is integrated to modem 4 and is connected to receive the output of amplifier 16. However, such a connection imposes letting amplifier 16 operate in a nominal mode while circuit 2 is at stand-by. The consumption of amplifier 16 thus is relatively high and, at stand-by, circuit 2 cannot comply with international standards such as the USB (Universal Serial Bus) standard, which especially determines the maximum consumption at stand-by. As an example, the consumption of an amplifier such as amplifier 16 is on the order of 100 mW/h while standard USB requires a stand-by consumption at most equal to 10 mW/h. It is thus impossible to have a circuit 2 with a low-consumption mode at stand-by.
To solve this problem, it may be devised to connect the start-of-communication signal detector directly on receive line 10, and to set amplifier 16 to a low-consumption mode. However, this solution is not retained since it is very expensive. Indeed, above all in the case where transmission line 6 is long, the signal received by amplifier 16 is sometimes very weak, reaching levels as low as 10 μV. Amplifier 16 must thus be a particularly sensitive amplifier, the forming of which is critical. This amplifier is normally directly connected to a transformer winding and its input impedance is provided according to this connection. If a signal detector circuit is added on the input of amplifier 16, the operation of this amplifier is disturbed, even if it is provided to disconnect the detector circuit when the amplifier is active. Indeed, in this last case, at least the stray capacitance of a switch ensuring said disconnection will be nevertheless added on the amplifier input.
It may also be devised to directly connect the start-of-communication signal detector to transmission line 6. This solution is also discarded because of its cost. Indeed, a significant electric isolation, on the order of 3500 V, imposed by international standards, must exist between transmission line 6 and any subscriber circuit. This isolation is performed by coupling means 12, which conventionally includes a transformer having a primary winding connected to transmission line 6, and first and second secondary windings respectively connected to transmit line 8 and to receive line 10. A detector directly connected to line 6 should exhibit the same electric isolation as coupling means 12, which would make the circuit too expensive.